Preheat oven to 180°C (350°F).
Grease a 12-cup muffin tray.
In a large bowl, whisk together eggs and milk.
Add spinach, bell peppers, onion, cheese, salt, pepper, and oregano.
Mix until well combined.
Pour the mixture evenly into the muffin cups, filling each about ¾ full.
Bake for 18–20 minutes or until the muffins are fully set and lightly golden.
Let them cool for 5 minutes before removing.
Place 2–3 egg muffins into each compartment of your silicone storage container.
Refrigerate or freeze for easy grab-and-go breakfasts.